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  A sequence of bits of a given fixed length. In this Standard, blocks consist of 128 bits, sometimes represented as arrays of bytes or words.
FIPS 197 [NIST FIPS 197-upd1] under Block

  A binary vector. In this document, the input and output of encryption and decryption operation are 64-bit block. The bits are numbered from left to right. The plaintext and ciphertext are segmented to k-bit blocks, k = 1, 8, 64.
NIST SP 800-20 under Block

  A subset of a bit string. A block has a predetermined length.
NIST SP 800-22 Rev. 1a under Block

  For a given block cipher, a bit string whose length is the block size of the block cipher.
NIST SP 800-38B under Block
NIST SP 800-38D under Block
NIST SP 800-38F

  A bit string whose length is the block size of the block cipher algorithm.
NIST SP 800-38C under Block

  In this Recommendation, a binary string, for example, a plaintext or a ciphertext, is segmented with a given length. Each segment is called a block. Data is processed block by block, from left to right.
NIST SP 800-67 Rev. 2 under Block

  A data structure containing a block header and block data.
NISTIR 8202 under Block